The present invention relates to an MOS transistor voltage clamping circuit, and particularly but not exclusively to n-channel MOS transistor voltage clamping circuits for use with MOS logic circuitry employed in domestic appliances such as washing machines and tumble driers.
In manufacturing domestic appliances it is desired to keep the control circuit and switches as simple and as inexpensive as possible consistent with reliable operation. The development of n-channel MOS transistors and integrated circuits has enabled low cost logic circuitry to be developed. In the case of switches, a problem arises in keeping the contacts clean without the need for periodic attention by a servicing engineer, accordingly line voltage is applied to the switch contacts and thereby enables the contacts to be self cleaning. The use of line voltage does present a problem to the circuit designer of how to couple line voltage to the input stage(s) of a logic circuit, since this voltage may have spikes of up to 1 kV superimposed thereon. If the logic circuit is fabricated as an integrated circuit another problem to be overcome is the effect of process variations on the parameters of the circuit elements.